I have the two 6V batteries out of our trailer while it's stored (too much of a hassle to charge the batteries at the storage location). (The batteries are original equipment Interstates from the trailer production Spring '16.)

I want to get an inexpensive charger to keep the batteries charged while they're at the house -- two questions:

-- Is there any problem in charging the two batteries in series with a 12V charger? (There are 6 & 12 volt chargers, but not as many as 12V only.)

-- Is there any problem in charging the two batteries in series with a 12V charger? (There are 6 & 12 volt chargers, but not as many as 12V only.)

No, and that would be the easiest thing to do. It will also ensure that exactly the same current goes through both batteries.

Having said that, my guess is that they would be somewhat better off being separately maintained... but I've never done that with mine (they've been connected for their entire working lives). I'm sure someone has a more authoritative opinion.

My two cents - since the batteries are already out of your camper, it might be simpler and cheaper to purchase two small single-battery maintainers than a single unit designed to maintain two batteries. Price them out and see what you find in your location. (Battery Tender, BatteryMINDer and Schumacher all get good reviews, and I'm currently using all three brands with good results, including a couple powered by solar panels.)

Moultrie also makes relatively inexpensive 6V battery chargers/maintainers for their outdoor products like game cameras and game feeders. Not sure how they would work on deep-cycle automobile-type batteries, but others might have insight into that aspect.

I know 2 x 6V = 1 x 12V, but my concern about charging 2 in series would be if somehow they were not equally discharged. Since the charger goes through multiple charging phases, how would that work? In any case, I decided on a charger that does both 6V and 12V.

I know 2 x 6V = 1 x 12V, but my concern about charging 2 in series would be if somehow they were not equally discharged.

Quote:

Originally Posted by padlin

How could they be not equally discharged?

It is true that two 6-volt batteries in series can get "out of synch", and have different levels of charge. Ideally this shouldn't happen with the batteries in series (so the same charging and discharging current goes through both), but in practice it happens because batteries are not perfect and internally discharge to a small extent. Any battery self-discharges slowly while in storage without any external connection, and each cell is doing this... at slightly different rates; two groups of cells (that is, two batteries) will self-discharge at different rates and so end up mismatched.

Cells - and batteries - in series get out of synch during charging, not just storage. Not all of the charge put into a battery is successfully stored by the battery, and even "identical" batteries won't perfectly match each other in charging efficiency, so they end up with different charge levels. During normal discharging in use the same sort of thing happens.

This isn't a big deal, and exactly the same thing happens between the cells of a battery (whether 6-volt or 12-volt). When a battery dies, it is often just one cell that has gone bad, but since they are all built into one case the whole battery has to be replaced. With two 6-volt batteries, the usual advice (which makes sense to me) is to treat them as if they were attached to each other: buy them as a set, and replace them together with another set.

So, charging the pair of 6-volt batteries together means just treating them as one big 12-volt battery (as others have already posted), and clearly that works. The question is whether or not there is a meaningful benefit to taking advantage of the opportunity to charge each set of three cells (one 6-volt battery) to the same state of charge and reduce the mismatch.

Quote:

Originally Posted by Tom and Sharon

Since the charger goes through multiple charging phases, how would that work?

It's the same as with any battery made up of multiple cells: the charger sees the total voltage, responds to that total, and the change between charging modes isn't quite at the ideal point for each individual cell. That's okay - even if there were one charger per cell normal chargers still wouldn't manage the charging ideally.